1. Field of the Invention
The present invention relate; generally to packaging and, more specifically, to a laminated chip scale package formed of a die and a micromachined silicon wafer segment or blank bonded to the active surface of the die. The package may be executed at the wafer level.
2. State of the Art
Packaging for semiconductor dies takes a variety of forms. Transfer-molded packages, comprising a filled polymer encompassing a die wire-bonded or otherwise electrically connected to a lead frame, are prevalent in today""s market. Other types of packaging, such as preformed ceramic or even metal packages, in which die are secured and then placed in electrical communication with package conductors, are also employed. Similarly, so-called xe2x80x9cglob-topxe2x80x9d encapsulation (with an epoxy, silicone gel, polyimide, and other organic, plastic and the like) of dies mounted and usually wire-bonded to a substrate such as a printed circuit board is also widely employed. Underfill of a flip-chip mounted to a substrate is also known in the art; such procedure may be followed by glob-topping the assembly. It has also been suggested to hermetically protect integrated circuits (dies) with a silicon-containing ceramic layer; see U.S. Pat. No. 5,481,135.
All of the foregoing packaging schemes, however, suffer from one or more deficiencies. For example, plastic packaging with lead frames and wire bonds is a multi-step process, wherein a defectively-performed individual step may compromise the quality of the end product if any individual step is deficient. Moreover, matching of the coefficients of thermal expansion (CTE) of the die, lead frame and encapsulant is virtually impossible, requiring additional structural features or process steps to accommodate thermally-induced stresses. Further, plastic packages do not provide a hermetic seal, e.g., are not effective to prevent the ingress of moisture to the package interior. Ceramic and metal packages provide hermetic protection, but are expensive and require as many if not more process steps as a transfer-molded plastic package. Glob-topping a die is relatively easy, but the resulting protection for the die and conductors is less than robust in comparison to other alternatives. Underfilling of a flip-chip connection followed by glob-topping is process-intensive and suffers from quality control constraints due to an inability to verify the integrity of the underfill. With the exception of ceramic and metal packages, all of the current packaging alternatives, including application of a ceramic layer to the surface of a die, may fail to provide a hermetic seal of any quality or repeatability for the die. Moreover, most current die packages are far more massive in both lateral and vertical extent than the die itself, thus absorbing valuable xe2x80x9creal estatexe2x80x9d on the substrate or other carrier to which the die is mechanically attached and electrically connected, and increasing the size of the external circuit in which the die is incorporated.
So-called xe2x80x9cdirectxe2x80x9d die attach (DDA) or xe2x80x9cdiscretexe2x80x9d or xe2x80x9cdirectxe2x80x9d die connect (DDC) configurations have been developed to facilitate the direct connection of one or more unpackaged or xe2x80x9cbarexe2x80x9d die to the next level of packaging. Such schemes may simply use a variation of a flip-chip die attach, may actually employ an intermediate substrate carrying more than one die to effect the connection to a carrier, or may use an xe2x80x9cedge-connectxe2x80x9d arrangement to mechanically and electrically connect vertically-oriented die to a carrier. These approaches, while meritorious from a space-saving standpoint, subject the bare die itself to potential damage during handling and execution of the die-connect, as the relatively delicate active surface of the die, with its active and passive devices as well as a myriad of conductive traces, is placed at risk. Moreover, configuring dies with a bond pad arrangement suitable for an edge-connect is no small feat, given the necessity of placement of all of the external connections for accessing the die at one edge thereof. Thus, some edge-connect approaches are a compromise of a true direct die connect by virtue of using a larger, conductor-carrying film or board to effect the edge connections.
In summary, state-of-the art packaging schemes fail to achieve reliable, substantially hermetic die protection on a size scale of the die itself, which the inventors herein term a xe2x80x9cchip scalexe2x80x9d package. Moreover, state-of-the-art packaging schemes fail to provide a technique to reliably effectuate a chip scale DDC with hermetic die protection.
The present invention comprises a chip scale package which may be fabricated at the wafer level, and which provides hermetic protection for the die. The invention may also be used to reroute bond pads for flip-chip direct die connect (DDC) and direct die attach (DDA) use, and the package structure itself is advantageous for the formation and use of solder or conductive epoxy balls or bumps in a flip-chip format. Further, the package is readily adaptable to the stacking of dies to form multi-die circuits.
In its elemental form, the package of the present invention comprises a bare semiconductor or integrated circuit die having a micromachined silicon segment or blank bonded to its face. With the exception of the bond pad locations, the active surface of the die is passivated with an insulative layer by formation of an oxide or nitride layer thereon, after which the silicon blank is aligned with and bonded to the die, micromachined apertures extending through the blank being aligned with the bond pads of the die. The exterior of the entire package is then passivated as, for example, by nitriding or oxidizing, after which the nitride is removed from the bond pads as known in the art. The package is, at that point, ready for wire-bonding or Tab Automated Bonding (TAB) conductor attach.
If desired, copper may be electrolessly plated onto the bond pads of the micromachined apertures as known in the art, or a multi-layer xe2x80x9csandwichxe2x80x9d coating of Cr, Cr/Cu and Cu formed over the bond pads and the adjacent walls of the apertures. This treatment of the package structure is then followed by stenciling or screen-printing of solder over and into the apertures, followed by reflow to form solder balls of a ball grid array, or BGA. Alternatively, conductive polymer bumps or columns may be stenciled or printed over and into the apertures to define conductive bumps, or bump-type contacts may be electrolessly plated into the apertures.
The package of the invention may be employed to move or reroute bond pad locations in several ways. For example, the die may be formed with circuit traces on its active or xe2x80x9cfacexe2x80x9d side, leading from the original bond pads at the die periphery or in a central row to alternate bond pad locations (in an area array, for example) accessed through the apertures of the blank. The original bond pad locations may also be deleted during die fabrication and circuit traces configured to lead to different bond pad locations. Alternatively, the back side of the blank may be used to repattern the bond pad pattern by stenciling of conductive traces extending from the bond pad locations of the die to new locations accessible through apertures formed in the blank. In yet another approach, the bond pad or terminal locations may be moved by accessing the die bond pads through micromachined apertures in the blank communicating with trenches micromachined in the face side of the blank, which trenches lead to new pad locations also formed in the blank""s face side. The trench surfaces and new bond pads may then be metallized for electrical communication. In such a manner, closely-spaced bond pads unsuitable for flip-chip bonding may be transformed into a wider pitch area array easily susceptible to employment in a flip-chip mounting scheme.
In yet another variation of the invention, the blank may be micromachined with a number of mutually parallel, extended grooves along and perpendicular to one edge of the package. The grooves, which may communicate directly with the bond pads of the die, or with rerouting traces on the face side of the die or the back side of the blank extending from original bond pad locations to new ones along one edge of the package, may be conductively coated to function as connectors when the package is xe2x80x9cpluggedxe2x80x9d transversely into a carrier having slots with mating conductive clips or other elements to receive and connect to the die of the package in a DDC assembly. Alternatively, the trenches may function merely as alignment elements for the clips, which contact the new bond pads associated with the trenches.
While the invention, and its many variations, have heretofore been discussed literally on a xe2x80x9cchipxe2x80x9d or die scale, it will be understood and appreciated by those of ordinary skill in the art that the invention may be most efficiently practiced in its method aspect on a wafer scale. That is to say, it is preferred that an entire wafer of active device dies be processed according to the invention in combination with a blank of wafer size, so that all method steps, including burn-in and testing of the xe2x80x9cmacroxe2x80x9d scale package including a multitude of packages, be performed before the packages are singulated. It is also contemplated that packages may be defined as comprising multiple, unseparated dies, or partial wafers, in combination with like-sized blanks and associated conductors to provide external access to the circuit with which the multiple dies are to be employed. Thus, a multi-chip module (MCM) such as a single in-line memory module (SIMM) may be replaced by a multi-die package according to the present invention. Similarly, processors which are to be paralleled may also be formed side-by-side in a single package with all required conductors. Further, packages according to the invention may be attached face-to-face against opposing sides of a conductor-carrying substrate, or face-to-face against each other, and employ suitable edge-connect structure for external connections to a circuit.